1. Field of the Invention
The present invention relates to an electromagnetic flowmeter which converts a flow rate of fluid to be measured into an electric signal and performs a signal processing on the electric signal to obtain and output a flow rate signal. In particular, the present invention relates to an electromagnetic flowmeter which transmits an electric signal of fluid to be measured which has a low conductivity to a converter via a shielded cable.
2. Description of the Related Art
FIG. 2 is a schematic diagram showing the configuration of a first electromagnetic flowmeter of a related art. A detector 1 includes an excitation circuit 10, excitation coils 11A, 11B, detection electrodes 13A, 13B, a conduit pipe 14 (not shown), a grounding electrode 19 (liquid-contacting electrode), etc.
The excitation circuit 10 supplies an excitation current If with a predetermined waveform and a predetermined frequency to the excitation coils 11A, 11B, and also outputs a timing signal T1 necessary for signal processing to a converter 12.
The excitation coils 11A, 11B apply a magnetic field B having a waveform and a frequency corresponding to the excitation current If, to fluid Q to be measured (measuring fluid Q). The detection electrodes 13A, 13B are fixed to the insulative conduit pipe 14 by being insulated from the measuring fluid Q. These detection electrodes 13A, 13B are connected to input terminals of preamplifiers 15A, 15B of the converter 12 via terminals TA, TB, respectively. The measuring fluid Q is grounded via the grounding electrode 19.
The electric signals being outputted from the output terminals of the preamplifiers 15A, 15B are subjected to differential processing in a differential amplifier 16, and the processing result is outputted to a signal processing circuit 17. The signal processing circuit 17 performs signal processing, and outputs the result of the signal processing to the output terminal 18 of the converter 12 as a flow rate signal. The timing signal T1 is used at the time of the signal processing in the converter 12.
FIG. 3 is a diagram showing the detailed configuration of the preamplifiers 15A, 15B of the electromagnetic flowmeter shown in FIG. 2. Since either of the preamplifiers 15A, 15B is configured similarly, the following explanation will be made based on the preamplifier 15A. In the figure, the detector 1 is shown only on the electrode 13A side.
In the figure, Vd represents an electromotive force detected by the detection electrode 13A. The gate G of a field-effect transistor Q2 is connected to the detection electrode 13A and also connected to a common voltage point COM via a series circuit in which resistors R5 and R6 are connected in series.
The source S of the field-effect transistor Q2 is connected to a power source VSS via a resistor R7 and also connected to a non-inverted input terminal (+) of an operational amplifier Q3. The drain D thereof is connected to a power source VDD via a resister S8. Thus, the field-effect transistor Q2 serves as a source follower.
The output terminal TC1 of the operational amplifier Q3 is connected to the drain D of the field-effect transistor Q2 via a capacitor C3. Further, the output terminal TC1 is connected to the inverted input terminal (−) of the operational amplifier Q3 and also connected to a connecting point between the resistors R5 and R6 via a capacitor C4. A bootstrap circuit BS1 for performing positive feedback on the input side includes these resistors R5, R6 and the capacitor C4.
Since the resistor R5 and the field-effect transistor Q2 makes a high-impedance circuit, the high-impedance circuit is surrounded by a shielding plate 20. The shielding plate 20 is connected to the output terminal TC1, whereby both the shielding plate and the output terminal are held at the same voltage.
In the aforesaid configuration, since the field-effect transistor Q2 serves as the source follower, the amplification degree of this transistor is represented by gm/[(1/R7)+gm], where gm represents a mutual conductance of the field-effect transistor Q2. Thus, the amplification degree of this transistor is almost 1 when the value of the resistor R7 is set to a large value.
Further, since the operational amplifier Q3 is configured as a voltage follower, the amplification degree of this transistor is also 1. Thus, since the amplification degree of the combination of the field-effect transistor Q2 and the operational amplifier Q3 is almost 1, this field-effect transistor and this operational amplifier are kept at the same voltage in view of alternating current.
Since the voltages of the gate G and the drain D of the field-effect transistor Q2 are the same, a capacitance CGD is not formed therebetween. Further, since the field-effect transistor Q2 serves as the source follower, the gate G and the source S thereof are kept at almost the same voltage in view of alternating current, so that a capacitance CGS is also not formed therebetween. In view of these facts, the input capacitance of the field-effect transistor Q2 is entirely removed.
Further, since the voltage at the output terminal of the operational amplifier Q3 is applied via the capacitor C4 to the connecting point between the resistors R5 and R6 which are connected between the gate G and the common voltage point COM, the voltage at the connecting point between the resistors R5 and R6 is substantially same as that of the gate G, so that a current does not flow through the resistor R5. Thus, impedance on the field-effect transistor Q2 side relative to the detection electrode 13A side is infinite.
Therefore, in the preamplifier 15A shown in FIG. 3, even when the inner resistance value of the detector 1 becomes high in such a case of measuring the measuring fluid Q having a low conductivity, etc., the preamplifier 15A of the converter 12 equivalently raises the input impedance of the preamplifier 15A by using the bootstrap circuit BS1 thereby to prevent attenuation of the signal at the time of receiving the electromotive force Vd.
In other words, the measuring fluid Q having a low conductivity can be measured without error by using the preamplifier 15A shown in FIG. 3.
FIG. 4 is a schematic diagram showing the configuration a second electromagnetic flowmeter of a related art. FIG. 4 is an example of another preamplifier 21 located at a corresponding position to the preamplifier 15A shown in FIG. 2. In this case, also the detector 1 is shown only on the detection electrode 13A side. This example shows a case in which the preamplifier 21 is connected via a shielded cable 23 to the detector 1 which has an inner resistor Rd between the grounding electrode 19 and the detection electrode 13A and generates the electromotive force Vd.
The shielded cable 23 is configured in a manner that a cable core 25 is covered by an inner conductor 24, the external portion of the inner conductor 24 is covered by an external conductor 26, and the external conductor 26 is grounded. A stray capacitance Cs1 is formed between the cable core 25 and the inner conductor 24, and a stray capacitance Cs2 is formed between the inner conductor 24 and the external conductor 26.
The detection electrode 13A is connected to the non-inverted input terminal (+) of an operational amplifier Q4 by the cable core 25 via the resistor R10. The output terminal TC2 of the operational amplifier Q4 is connected to the inverted input terminal (−) of the operational amplifier Q4, whereby the operational amplifier Q4 constitutes a voltage follower.
The output terminal TC2 is connected to the common voltage point COM via a series circuit of a resistor R11 and a resistor R12 of which resistance value is variable. The connecting point between the resistor R11 and the resistor R12 is connected to the inner conductor 24. Thus, the positive feedback is made with respect to the inner conductor 24, whereby the inner conductor 24 is shield-driven.
Supposing that the feedback ratio of the positive feedback is 1, for example, voltage of the cable core 25 and the voltage of the inner conductor 24 become the same, so that the influence of the stray capacitance Cs1 existing between the cable core 25 and the inner conductor 24 can be eliminated. Thus, a cable can be laid over a longer length.
As described above, since the preamplifier 21 shown in FIG. 4 can prevent the electromotive force Vd from being attenuated due to the stray capacitance Cs1 of the cable, it is effective to use the preamplifier in the case of measuring the measuring fluid which has a high conductivity and a large inner resistor Rd.
The related art references are JP-A-6-241856, JP-A-2004-219372, JP-A-5-172602, JP-A-5-231890, JP-A-7-27580, JP-A-2004-138457, for example.
In the case of measuring the measuring fluid having a low conductivity, the fluid having the low conductivity can be measured without error by the method of avoiding the attenuation of the electromotive force by raising the input impedance of the preamplifier in the case of FIG. 3, or by the method of avoiding the attenuation of the electromotive force due to the stray capacitance of the cable connecting the detector and the converter in the case of FIG. 4, respectively.
However, actually, in order to measure the fluid having a low conductivity by connecting the detector and the converter by the cable, it is required to raise the input impedance of the preamplifier and simultaneously to avoid the attenuation of the electromotive force due to the stray capacitance of the cable. In this case, it is necessary to realize such the two requirements by a single preamplifier.